1. Field of the Invention
The present invention relates to a display control device and method for a flat panel display having a plurality of horizontal pixel lines and, more particularly, to a display control device and method for driving the flat panel display by a video signal of a scheme including horizontal picture signals larger in number than the horizontal pixel lines.
2. Description of the Related Art
In recent years, flat panel displays such as a liquid crystal display are used worldwide. With this advance, the liquid crystal display must be arranged to be compatible with all the video information of various media such as NTSC (National Television System Committee), EDTV (Extended Definition Television), PAL (Phase Alternation by Line), a high-vision broadcast, and a car navigation system.
For example, an NTSC video signal includes 480 horizontal picture signals per frame (240 signals per field), whereas a PAL video signal includes 512 horizontal picture signals per frame (256 signals per field). For example, when the PAL video signal is supplied to a liquid crystal display having horizontal pixel lines (horizontal scanning lines) whose number is compatible with the NTSC video signal, the horizontal picture signals of the PAL video signal cannot be properly assigned to the horizontal pixel lines of the liquid crystal display, and it is difficult to perform a normal display.
This problem can be solved by converting the PAL video signal into the NTSC video signal in the display control device for the liquid crystal display. However, a complicated structure is required for this signal conversion processing in order to perform a normal display, resulting in an increase in manufacturing cost of the display control device. For this reason, a conventional display control device is constituted to thin out the horizontal picture signals of the PAL video signal at a predetermined rate. In this case, since the structure of the display control device is simplified, the increase in manufacturing cost can be suppressed.
The horizontal pixel lines of the liquid crystal display are sequentially selected by a Y-driver circuit arranged in the display control device. A typical Y-driver circuit comprises a shift register constituted by a plurality of flip-flops. In this case, the Y-driver circuit receives a reference clock signal A having a frequency corresponding to a horizontal scanning period, as a shift clock signal CPV. In response to the shift clock signal CPV, each flip-plop outputs a scanning start pulse and shifts the start pulse in the next stage. A scanning signal obtained by shifting the level of an output signal from the flip-flop holding the start pulse is supplied to one of of the Y1, Y2, . . . horizontal pixel lines. Therefore, each horizontal picture signal is supplied to the horizontal pixel line selected by the scanning signal and displayed thereon. The above-described thinning processing is performed by generating a scanning inhibit signal GINH every predetermined number of horizontal scanning periods and masking the reference clock signal A and the scanning signal supplied to, e.g., the line Y1 of the horizontal pixel lines by the scanning inhibit signal GINH.
The Y-driver circuit is normally mounted as an individual IC module on the substrate of the liquid crystal display. For this reason, the supply timing of the scanning inhibit signal GINH to the Y-driver circuit does not exactly coincide with the supply timing to the generating circuit of the shift clock signal CPV. On the other hand, the leading and trailing edge timings of the scanning signal are delayed by a response time tpd1, which varies depending on the circuit characteristics of the shift register. If the response time tpd1 of the shift register exceeds a delay time tpd2 of the scanning inhibit signal GINH supplied to the shift register, an interference pulse short in duration is output as a scanning signal, as shown in FIG. 1. This interference pulse changes the pixel potential of the corresponding horizontal pixel line and affects a display image to generate, e.g., an unnecessary stripe. This influence becomes more serious when the number of pixels whose potentials must be set within one horizontal scanning period is increased with an increase in size of the liquid crystal display.